unAPI, the lightweight mechanism for retrieving objects—or their metadata—in different formats, reached version 1 a few weeks ago, and has been implemented in HubMed and other places already. While unAPI links are designed to be machine-usable, →this Greasemonkey script← makes them visible in the browser, showing all the different export formats available for an item. Try it on this page (click the unapi link to show the individual formats).
Xiaoming Liu made an earlier version of this script, and Dan Chudnov before that.
Update: now it's a drop-down menu.